In precision machines such as machine tools, coordinate measuring machines, and other like machines, gas bearings are frequently used to permit relatively frictionless movement of portions of the machine, including a carriage, along machine rails. Although the rails along which the carriage, or other portion of the machine, moves typically are rendered as smooth and as straight as possible, and the opposed sides thereof are machined to be as parallel to one another as possible, it is very difficult to make the sides of the rails exactly parallel along their entire length. Also, imperfections in the rails and in the machine parts can develop through use, and as a result of temperature variations which cause temperature induced strains in the rails or in the carriage. In addition, it may be prohibitively-pensive to machine the rails and the machine parts to the required degree of precision and to maintain them in that condition. Therefore, sometimes the rails in such precision machines are not as straight as desired or their sides are not as parallel as desired. The longer the path of travel, or the longer the rail, the more likely it is that imperfections will be found, and the more likely it is that such imperfections will cause unacceptable errors. In precision machines, even very minor imperfections in the rails, or even in very minor misalignments in the machine parts can produce significant errors
Often, it is desirable to brake the carriage and lock it in place with regard to the rail while a measurement or work is being performed on the workpiece. In prior art machines, the flow of gas must be maintained during this braking procedure. Interruption of the gas supply to one or more of the air bearings would cause the bearing shoe to directly engage the rail and to brake the carriage. However, if the gas flow is interrupted, he force applied by the gas pressure to the shoe through the carriage would change. This change in the applied force on one side would cause a shift in the balance of forces that previously existed between opposed gas bearings. As a result, the carriage would shift along with the shoe with respect to the rail, thus temporarily throwing the carriage out of alignment with regard to the other parts of the machine, and causing distortion of the machine geometry. In precision machining, and in coordinate measuring machines, such distortion can cause unacceptable errors in the machined product or in the measurement being made respectively. To maintain the desired alignment of the carriage on the rail, it is desirable that a constant preload be applied through opposed gas bearings to the bearing shoe at all times, whether the carriage is moving along the rail, or whether it is stationary, and whether or not the gas bearing has gas flowing through it or not.
Several attempts have been made to solve the problem of maintaining the desired alignment of the carriage on the rails when gas bearings are used, even where there are imperfections in the rails Examples of such solutions using gas bearings are found in U.S. Pat. Nos. 3,578,827; 3,717,392; 4,643.590; 4,378,134; and 4,174,136. All of these prior art devices are mechanically complex and are subject to failure at multiple locations.
It is a general object of the present invention to provide a gas bearing which maintains a constant preload force on the bearing pad, when imperfections, including non-parallel sides, are encountered in the rails.
It is another object of the present invention to provide a gas bearing which permits braking of the carriage by interruption of the gas flow to the bearing while maintaining a constant preload force on the bearing shoe and a precise alignment of the carriage on the rail.
It is another further object of the present invention to provide a simple, efficient, economical and reliable gas bearing which can accommodate the imperfections in the rails and maintain a precise alignment of the carriage on the rails.
It is also another further object of the present invention to provide a gas bearing which provides a constant preload force whether or not gas is being supplied to the bearing.